In the present paper the terahertz time-domain spectroscopy (THz-TDS) was used to measure the absorption spectra of L-, D-, DL-α-alanine at room temperature. A number of well-resolved absorption peaks were observed in the range of frequencies from 0.3 to 3.1 THz, which showed large differences in the spectra between the enantiomers(L-, D-alanine) and the racemic compound(DL-alanine). In parallel with the experimental study, the computed vibrational spectra were also obtained by using first principles calculations based on the density functional theory(DFT), which were in good agreement with the experimental data. Results show that the absorption spectra in terahertz region originate from the collective vibration based on hydrogen bonds.